The present invention relates to a tuning fork flexural crystal vibration device, a crystal vibrator incorporating the crystal vibration device, and a crystal oscillator incorporating the crystal vibration device or the crystal vibrator.
A crystal vibrator or crystal oscillator incorporating a tuning fork flexural crystal vibration device as one of electronic components is mounted and used as a reference signal source or clock signal source in an electronic device such as a computer, a cellular phone, or a compact information device. Strong demands for downsizing, lower profile, and cost reduction have been conventionally imposed on the crystal vibrator and crystal oscillator. A conventional tuning fork flexural crystal vibration device will be described below with reference to the accompanying drawings.
Referring to FIGS. 9A, 9B, and 10, in a tuning fork flexural crystal vibration device 100, various types of electrodes, e.g., vibration electrodes, frequency adjustment electrodes, external connection electrodes to electrically connect to a packing container are formed on the surface of a crystal piece 110 having a tuning fork-like outer shape when viewed from the top. The tuning fork flexural crystal vibration device 100 has a thickness of about 100 μm and roughly comprises a base 101 and first and second vibration arms 102 and 103 protruding from one side of the base 101 in the same direction. The outer shape of the crystal piece 110 of the tuning fork flexural crystal vibration device 100 is generally formed by photolithography and chemical etching.
The first vibration arm 102 is provided with a first groove 104 having an opening in the front main surface of the arm, with the long side of the opening extending along the longitudinal direction of the first vibration arm 102. The second vibration arm 103 is provided with a second groove 106 having an opening in the front main surface of the arm, with the long side of the opening extending along the longitudinal direction of the second vibration arm 103. Each of these grooves has a depth of about 60 μm.
Referring to FIGS. 9A and 9B, an electrode 121 is mainly formed in the first groove 104 in the front main surface of the first vibration arm 102, and an electrode 122 is formed on the rear main surface of the first vibration arm 102. The two electrodes are electrically connected to each other. In addition, the electrodes 121 and 122 are electrically connected to the side surface electrodes (not shown) provided on the two side surfaces of the second vibration arm 103. The electrodes 121 and 122 are lead to an external connection electrode 124 via a wiring 123, thereby forming one terminal network.
On the other hand, the side surface electrodes (not shown) provided on the two side surfaces of the first vibration arm 102 are electrically connected to each other. These electrodes are mainly electrically connected to an electrode 125 in the second groove 106 in the front main surface of the second vibration arm 103 and an electrode 126 on the rear main surface, and are lead to an external connection electrode 128 via a wiring 127, thereby forming one terminal network. Consequently, two heteropolar terminal networks are formed in the tuning fork flexural crystal vibration device 100.
An alternating voltage is applied between the two terminal networks. In a momentary state, for example, the two side surface electrodes of the first vibration arm 102 are set at a + (positive) potential, and the electrodes 121 and 122 are set at a − (negative) potential. An electrical field is generated from + to −. In the second vibration arm 103, the polarities of the respective electrodes are reversed to those of the respective electrodes provided on the first vibration arm 102. These electrical fields generate expansion and contraction in the vibration arms 102 and 103 made of a crystal material to flex them. Providing the grooves 104 and 106 in the vibration arms 102 and 103, respectively, can reduce the crystal impedance (to be referred to as the CI hereinafter) of the tuning fork flexural crystal vibration device to a value as small as 100 kΩ or less (see Japanese Patent Laid-Open No. 53-93792 (reference 1) and Japanese Patent Laid-Open No. 56-65517 (reference 2)).
Another example of the conventional tuning fork flexural crystal vibration device is a device provided with a groove having an opening in the front main surface of each vibration arm and a groove having an opening in the rear main surface of each vibration arm, with the bottom surfaces of the two grooves facing each other in each vibration arm (see Japanese Patent Laid-Open No. 2004-297343 (reference 3) and Japanese Patent Laid-Open No. 2004-129181 (reference 4) in addition to references 1 and 2).
The tuning fork flexural crystal vibration device 100 described above or the like is mounted in a recess portion which is formed in an almost rectangular parallelepiped packing container made of an insulating material and has an opening in one main surface of the packing container. This mounted tuning fork flexural crystal vibration device is electrically connected to a plurality of external connection electrodes formed on the outer bottom surface of the packing container, and the recess portion in which the tuning fork flexural crystal vibration device is mounted is hermetically sealed by covering the opening of the recess portion with a cover member, thereby forming a crystal vibrator as an electronic component (see reference 3).
A crystal vibration device has a characteristic that the vibration frequency changes with a change in temperature (to be referred to as a “temperature characteristic” hereinafter). A temperature characteristic graph representing the relationship between temperature and frequency deviation amount in a tuning fork flexural crystal vibration device which flexurally vibrates exhibits a quadratic curve (parabola) with an upward convex shape having a predetermined temperature as a peak temperature.
In general, the temperature characteristic of a crystal vibration device or the like is expressed with reference to +25° C. A crystal vibration device, crystal vibrator, or crystal oscillator is required to have a temperature characteristic that, for example, a desired frequency deviation or less is obtained in the range of +60° C. on the high-temperature side to −10° C. on the low-temperature side, and frequency changes occur in a balanced manner on the high-temperature side and the low-temperature side.
However, the tuning fork flexural crystal vibration device 100 having the grooves with the above shapes and the crystal vibrator or crystal oscillator incorporating the tuning fork flexural crystal vibration device have a temperature characteristic with its peak temperature deviating from +25° C., as shown in FIG. 8. FIG. 8 shows a temperature characteristic with a peak temperature of +20° C. (the broken line portion) as an example of the temperature characteristic of the tuning fork flexural crystal vibration device 100. In this case, since the peak temperature deviates from the reference temperature to the low-temperature side, the frequency deviation amount on the high-temperature side becomes larger than that on the low-temperature side. That is, the temperature characteristic is out of balance with reference to +25° C. In addition, when, for example, a criterion for non-defective products is a deviation amount of −40 [ppm], a frequency deviation becomes equal to or more than a desired frequency deviation on the high-temperature side, and hence the device may be determined as defective.
In a tuning fork flexural crystal vibration device in which grooves are formed in the front and rear main surfaces of each vibration arm with the bottom surfaces of the two grooves facing each other, it is not easy to form grooves with the bottom surfaces of the two grooves accurately facing each other. This formation process requires complicated manufacturing steps, and hence may lead to low productivity.